Transportation ForecastingThe Four Step Model

Norman W. Garrick

Transportation ForecastingWhat is it?

Transportation Forecasting is used to estimate the number of travelers or vehicles that will use a given transportation facility in the future

The conventional approach to transportation forecasting is based on what is commonly know as the ‘four step model’

Norman W. Garrick

The Four Step Model

• Trip GenerationEstimates the number of trips from given origins and destinations

• Trip DistributionDetermines the destination for each trip from a given origin

• Mode ChoiceDetermines the mode choice for each trip

• Route AssignmentDetermines the specific route for each trip

Norman W. Garrick

Transportation ForecastingWhat is it used for?

Transportation forecasting models have often been used in a ‘predict and provide’ mode to determine the highway capacity needed in response to predicted growth in traffic volumes.

Often, in this case, the full impact of changes in land use is not properly taken into account and the resulting induced traffic is over looked.

Norman W. Garrick

Transportation ForecastingScenario Planning

If transportation forecasting models are adjusted to properly account for the impact of changes in land use patterns they are useful for evaluating the implications of different transportation choices. In other words, they can be used for so called ‘scenario planning’.

Norman W. Garrick

Scenario Planning ExampleTolls in Connecticut

What would be the impacts of adding tolls to I-95 and the Merritt Parkway in Fairfield County?

Norman W. Garrick

Scenario Planning ExampleTolls in Connecticut

Why is this information needed?What are some potential impacts?

Norman W. Garrick

Possible ChangesDue to Tolls in Connecticut

Short Term Change Long Term ChangeTrip Generation

Trip Distribution

Mode Share

Trip Assignment

Norman W. Garrick

Possible ChangesDue to Tolls in Connecticut

Short Term Change Long Term ChangeTrip Generation Negligible Substantial

(depending on land use changes)

Trip Distribution

Mode Share

Trip Assignment

Norman W. Garrick

Possible ChangesDue to Tolls in Connecticut

Short Term Change Long Term ChangeTrip Generation Negligible Substantial

(depending on land use changes)

Trip Distribution Negligible Substantial (depending on land use changes)

Mode Share

Trip Assignment

Norman W. Garrick

Possible ChangesDue to Tolls in Connecticut

Short Term Change Long Term ChangeTrip Generation Negligible Substantial

(depending on land use changes)

Trip Distribution Negligible Substantial (depending on land use changes)

Mode Share Some switch to rail

Significant(depending on land use changes)

Trip Assignment

Norman W. Garrick

Possible ChangesDue to Tolls in Connecticut

Short Term Change Long Term ChangeTrip Generation Negligible Substantial

(depending on land use changes)

Trip Distribution Negligible Substantial (depending on land use changes)

Mode Share Some switch to rail

Significant(depending on land use changes)

Trip Assignment SomeSwitch to rail and to toll free roads

Significant(depending on land use changes)

The purpose of Transportation Forecastingis to quantify these changes as accurately as possible